IV 



VERMES NERVOUS SYSTEM 



217 



imbedded in, or directly under the epithelium (Carinina, Carinella), or they are 

 enclosed in the musculature of the body wall (Cephalothrix, Cerebratulus}, or they 

 lie on the inner side of the musculature (AmpMporus). 



FIG. 141. Anterior end of the body of a Nemertian from the side, diagrammatic (partly after 

 Hubrecht). g, Brain ; re, proboscis commissure ; dn, medio-dorsal nerve ; Iso, cerebral lobe of the 

 lateral organ ; sn, lateral nerve ; mo, mouth ; r, proboscis ; d, intestine ; nr, nerve rings, commis- 

 sures between lateral nerves and the medio-dorsal nerve ; TO, proboscidal aperture. 



In Malacobdella ganglionic swellings were formerly erroneously described as 

 occurring in the course of the longitudinal trunks. 



Nemathelminths. The nervous system of the Nematoda (Fig. 

 142) consists of a ring surrounding the oesophagus, the sides of which 

 often swell out into a ganglion. A dorsal and a ventral longitudinal 

 nerve arise out of the ring, and these run in the middle line to the 

 posterior end of the body. The two nerves are connected together by 

 transverse commissures which run under the cuticle outside the muscu- 

 lature. The transverse commissures of the right and left sides of the 

 body do not exactly correspond. Numerous nerves proceed forwards 

 from the oesophageal ring towards the apex of the head. 



Attempts have been made to trace back the nervous system of the Nematoda to 

 that of the Platodes in the following way. By union of the dorsal longitudinal 

 trunks in the middle line the medio-dorsal nerve of the Nematoda arose, and by 

 the union of the ventral longitudinal trunks the medio-ventral, which in young 

 Ascaridce and in a Plectus is still distinctly paired. The dorsal half of the Nematodan 

 oesophageal ring answers to the brain of the Platodes with its often distinct lateral 

 swellings. The ventral half of the cesopliageal ring answers to the proximal portions 

 of the ventral longitudinal trunks, still separated in the region of the oesophagus, 

 but fused in the ventral middle line to form the medio-ventral nerve. The lateral 

 nerves which run a short distance at the anterior end of the Nematoda are considered 

 to be the remains of the lateral longitudinal nerves. The transverse commissures 

 of the Nematoda correspond with the transverse commissures which connect the 

 various longitudinal nerves of the Platodes. We prefer, however, to compare the 

 nervous system of the Nematoda with that of the Nemertina, because there we have 

 an unpaired dorso-median nerve, and the nerves corresponding with the outer lateral 

 longitudinal nerves of the Platodes are wanting. The lateral swellings of the brain 

 also are developed more clearly as lateral ganglia connected by a transverse com- 

 missure than is the case in the Platodes. 



